A bundle of tube shaped semi-permeable membranes, when perfused with tissue culture medium simulates the in vivo capillary bed. Isolated cells injected into the extracapillary space receive nutrients and have cell products removed by diffusion through the capillary walls. In this nearly physiologic environment the cells form solid masses of tumor. The effect of various hormones on these in vitro solid tumors is being studied to determine the mechanisms of hormone-dependent tumor growth and regression. Specific hormone receptors are being studied and isolated to complement these studies.